@article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106873, title ="Sulfate sulfur isotopes and major ion chemistry reveal that pyrite oxidation counteracts CO₂ drawdown from silicate weathering in the Langtang-Trisuli-Narayani River system, Nepal Himalaya", author = "Kemeny, P. C. and Lopez, G. I.", journal = "Geochimica et Cosmochimica Acta", volume = "294", pages = "43-69", month = "February", year = "2021", doi = "10.1016/j.gca.2020.11.009", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201202-083406031", note = "© 2020 Elsevier Ltd. \n\nReceived 14 April 2020, Revised 2 November 2020, Accepted 11 November 2020, Available online 19 November 2020. \n\nP.C.K. is supported by the Fannie and John Hertz Foundation Cohan-Jacobs and Stein Families Fellowship. This research was conducted with government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. This research was supported by the US National Science Foundation (grants 1349858 and 1834492). N.F.D. is grateful to the Linde Center for support. The Caltech Environmental Analysis Center is supported by the Linde Center and the Beckman Institute at Caltech. This research was also supported by the German Research Foundation DFG through the Cluster of Excellence ‘CliSAP’ (EXC177), Universität Hamburg. The authors acknowledge the Department of Hydrology and Meteorology (DHM), Government of Nepal, for discharge measurements. Initial computing costs were covered by startup research funds provided by Caltech to F. Tissot. T. Jappinen and P. Bartsch helped with logistics and analysis. The authors are grateful to W. Fischer for helpful conversations and to A. Philips and P. Mateo for advice on the design of figures. We thank the associate editor and three anonymous reviewers for providing insightful comments. \n\nData availability: The measurements of δ³⁴S_(SO₄) and dissolved major ion concentrations described in this article are available as supplementary material. \n\nThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.", revision_no = "33", abstract = "Drawdown of atmospheric carbon dioxide (CO₂) due to silicate weathering in the Himalaya has previously been implicated in Cenozoic cooling. However, over timescales shorter than that of the removal of marine sulfate (SO₄²⁻), the oxidation of pyrite (FeS₂) in weathering systems can counteract the alkalinity flux of silicate weathering and modulate pCO₂. Here we present evidence from ³⁴S/³²S isotope ratios in dissolved SO₄²⁻ (δ³⁴S_(SO₄)), together with dissolved major ion concentrations, that reveals FeS₂ oxidation throughout the Langtang-Trisuli-Narayani River system of the Nepal Himalaya. River water samples were collected monthly to bimonthly throughout 2011 from 16 sites ranging from the Lirung Glacier catchment through the Narayani River floodplain. This sampling transect begins in the High Himalayan Crystalline (HHC) formation and passes through the Lesser Himalayan (LH) formation with upstream influences from the Tethyn Sedimentary Series (TSS). Average δ³⁴S_(SO₄) in the Lirung Glacier outlet is 3.6‰, increases downstream to 6.3‰ near the confluence with the Bhote Kosi, and finally declines to −2.6‰ in the lower elevation sites. Using new measurements of major ion concentrations, inversion shows 62–101% of river SO₄²⁻ is derived from the oxidation of sulfide minerals and/or organic sulfur, with the former process likely dominant. The fraction of H₂SO₄-driven weathering is seasonally variable and lower during the monsoon season, attributable to seasonal changes in the relative influence of shallow and deep flow paths with distinct residence times. Inversion results indicate that the primary control on δ³⁴S SO₄ is lithologically variable isotope composition, with the expressed δ³⁴S value for the LH and TSS formations (median values −7.0–0.0‰ in 80% of samples) lower than that in the HHC (median values −1.7–6.2‰ in 80% of samples). Overall, our analysis indicates that FeS₂ oxidation counteracts much of the alkalinity flux from silicate weathering throughout the Narayani River system such that weathering along the sampled transect exerts minimal impact on pCO₂ over timescales >5–10\u202fkyr and <10\u202fMyr. Moreover, reanalysis of prior datasets suggests that our findings are applicable more widely across several of the frontal Himalayan drainages.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/105466, title ="The Dissolution Rate of CaCO₃ in the Ocean", author = "Adkins, Jess F. and Naviaux, John D.", journal = "Annual Review of Marine Science", volume = "13", pages = "57-80", month = "January", year = "2021", doi = "10.1146/annurev-marine-041720-092514", issn = "1941-1405", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200922-074257620", note = "© 2021 by Annual Reviews. \n\nFirst published as a Review in Advance on September 18, 2020. \n\nWe thank the National Science Foundation for its support of this research via awards OCE-1220600, OCE-1559215, OCE-1834492, OCE-1559004, OCE-1220302, and OCE-1834475. We have also benefited from financial support from the Linde Center for Global Environmental Science at Caltech, the Change Happens Foundation, and the Grantham Foundation. Little of this work would have been possible without the technical support of Nick Rollins and intellectual inspiration from Jonathan Erez. Conversations with many colleagues over the years have helped shape our understanding of carbonate dissolution kinetics, but we are especially indebted to Burke Hales and Henry Teng for insightful comments and criticisms along the way. \n\nThe authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.", revision_no = "16", abstract = "The dissolution of CaCO₃ minerals in the ocean is a fundamental part of the marine alkalinity and carbon cycles. While there have been decades of work aimed at deriving the relationship between dissolution rate and mineral saturation state (a so-called rate law), no real consensus has been reached. There are disagreements between laboratory- and field-based studies and differences in rates for inorganic and biogenic materials. Rates based on measurements on suspended particles do not always agree with rates inferred from measurements made near the sediment–water interface of the actual ocean. By contrast, the freshwater dissolution rate of calcite has been well described by bulk rate measurements from a number of different laboratories, fit by basic kinetic theory, and well studied by atomic force microscopy and vertical scanning interferometry to document the processes at the atomic scale. In this review, we try to better unify our understanding of carbonate dissolution in the ocean via a relatively new, highly sensitive method we have developed combined with a theoretical framework guided by the success of the freshwater studies. We show that empirical curve fits of seawater data as a function of saturation state do not agree, largely because the curvature is itself a function of the thermodynamics. Instead, we show that models that consider both surface energetic theory and the complicated speciation of seawater and calcite surfaces in seawater are able to explain most of the most recent data. This new framework can also explain features of the historical data that have not been previously explained. The existence of a kink in the relationship between rate and saturation state, reflecting a change in dissolution mechanism, may be playing an important role in accelerating CaCO₃ dissolution in key sedimentary environments.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/105565, title ="A Mechanistic Study of Carbonic Anhydrase Enhanced Calcite Dissolution", author = "Dong, Sijia and Berelson, William M.", journal = "Geophysical Research Letters", volume = "47", number = "19", pages = "Art. No. e2020GL089244", month = "October", year = "2020", doi = "10.1029/2020gl089244", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200925-135425366", note = "© 2020 American Geophysical Union. \n\nIssue Online: 29 September 2020; Version of Record online: 29 September 2020; Accepted manuscript online: 21 September 2020; Manuscript accepted: 14 September 2020; Manuscript revised: 08 September 2020; Manuscript received: 11 June 2020. \n\nThis work was supported by the National Science Foundation (NSF) Ocean Acidification grants (OCE1220600, OCE1220302 and OCE 1559004) and the University of Southern California (USC) Dornsife Doctoral Fellowship. We thank Adam V. Subhas for his helpful discussions in preparing the experiments and the manuscript. \n\nData Availability Statement: Data set for this research is available in this in‐text data citation reference: Dong, S., Berelson, W., Teng, H., Rollins, N., Pirbadian, S., El‐Naggar, M., Adkins, J. (2020). Step velocities during calcite dissolution in seawater with and without carbonic anhydrase, version 1.0. Interdisciplinary Earth Data Alliance (IEDA). https://doi.org/10.26022/IEDA/111627. Accessed 2020‐09‐08.", revision_no = "23", abstract = "Carbonic anhydrase (CA) has been shown to promote calcite dissolution (Liu, 2001, https://doi.org/10.1111/j.1755-6724.2001.tb00531.x; Subhas et al., 2017, https://doi.org/10.1073/pnas.1703604114), and understanding the catalytic mechanism will facilitate our understanding of the oceanic alkalinity cycle. We use atomic force microscopy (AFM) to directly observe calcite dissolution in CA‐bearing solution. CA is found to etch the calcite surface only when in extreme proximity (~1 nm) to the mineral. Subsequently, the CA‐induced etch pits create step edges that serve as active dissolution sites. The possible catalytic mechanism is through the adsorption of CA on the calcite surface, followed by proton transfer from the CA catalytic center to the calcite surface during CO2 hydration. This study shows that the accessibility of CA to particulate inorganic carbon (PIC) in the ocean is critical in properly estimating oceanic CaCO3 and alkalinity cycles.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/105004, title ="The ECCO‐Darwin Data‐Assimilative Global Ocean Biogeochemistry Model: Estimates of Seasonal to Multidecadal Surface Ocean pCO₂ and Air‐Sea CO₂ Flux", author = "Carroll, D. and Menemenlis, D.", journal = "Journal of Advances in Modeling Earth Systems", volume = "12", number = "10", pages = "Art. No. e2019MS001888", month = "October", year = "2020", doi = "10.1029/2019ms001888", issn = "1942-2466", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200818-132613563", note = "© 2020 The Author(s). This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. \n\nAccepted manuscript online: 26 July 2020; Manuscript accepted: 23 July 2020; Manuscript revised: 15 July 2020; Manuscript received: 03 September 2019. \n\nResearch was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. D. C., D. M., and H. Z. were supported by NASA Biological Diversity, Physical Oceanography, and Modeling, Analysis, and Prediction Programs. M. M. G. and D. S. S. were supported by NASA ROSES‐2017 Grant/Cooperative Agreement NNX15AG92G. S. D. was supported by NASA‐IDS Grant 80NSSC17K0561. J. M. L. was supported by U.S. National Science Foundation grant OCE‐1259388. High‐end computing resources were provided by the NASA Advanced Supercomputing (NAS) Division of the Ames Research Center. The authors acknowledge ideas and advice from the participants in the Satellites to the Seafloor workshop organized by the W. M. Keck Institute for Space Studies. \n\nData Availability Statement: ECCO‐Darwin model fields are available at the website (https://data.nas.nasa.gov/ecco). Platform‐independent instructions for running ECCO‐Darwin simulations are available at the website (https://zenodo.org/badge/doi/10.5281/zenodo.3829965.svg). Copyright 2020 California Institute of Technology. U.S. Government sponsorship acknowledged. All rights reserved.", revision_no = "38", abstract = "Quantifying variability in the ocean carbon sink remains problematic due to sparse observations and spatiotemporal variability in surface ocean pCO₂. To address this challenge, we have updated and improved ECCO‐Darwin, a global ocean biogeochemistry model that assimilates both physical and biogeochemical observations. The model consists of an adjoint‐based ocean circulation estimate from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium and an ecosystem model developed by the Massachusetts Institute of Technology Darwin Project. In addition to the data‐constrained ECCO physics, a Green's function approach is used to optimize the biogeochemistry by adjusting initial conditions and six biogeochemical parameters. Over seasonal to multidecadal timescales (1995–2017), ECCO‐Darwin exhibits broad‐scale consistency with observed surface ocean pCO₂ and air‐sea CO₂ flux reconstructions in most biomes, particularly in the subtropical and equatorial regions. The largest differences between CO₂ uptake occur in subpolar seasonally stratified biomes, where ECCO‐Darwin results in stronger winter uptake. Compared to the Global Carbon Project OBMs, ECCO‐Darwin has a time‐mean global ocean CO₂ sink (2.47 ± 0.50 Pg C year⁻¹) and interannual variability that are more consistent with interpolation‐based products. Compared to interpolation‐based methods, ECCO‐Darwin is less sensitive to sparse and irregularly sampled observations. Thus, ECCO‐Darwin provides a basis for identifying and predicting the consequences of natural and anthropogenic perturbations to the ocean carbon cycle, as well as the climate‐related sensitivity of marine ecosystems. Our study further highlights the importance of physically consistent, property‐conserving reconstructions, as are provided by ECCO, for ocean biogeochemistry studies.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/105327, title ="Variability in Sulfur Isotope Records of Phanerozoic Seawater Sulfate", author = "Present, Theodore M. and Adkins, Jess F.", journal = "Geophysical Research Letters", volume = "47", number = "18", pages = "Art. No. e2020GL088766", month = "September", year = "2020", doi = "10.1029/2020gl088766", issn = "0094-8276", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200911-133129558", note = "© 2020 American Geophysical Union. \n\nIssue Online: 17 September 2020; Version of Record online: 17 September 2020; Accepted manuscript online: 02 September 2020; Manuscript accepted: 29 August 2020; Manuscript revised: 27 August 2020; Manuscript received: 14 May 2020. \n\nWe thank Caltech DocuServe for obtaining many of the publications containing the compiled data, and John Grotzinger and Joe Kirschvink for thoughtful advising and feedback. Constructive reviews by Akshay Mehra and Julia Wilcots were greatly appreciated. \n\nData Availability Statement: No new data were collected for this study. Data sets compiled for this research are tabulated in the supporting information and referenced below, and the compiled data are deposited in a freely accessible Open Science Framework repository available in Present et al. (2020).", revision_no = "30", abstract = "The δ³⁴S of seawater sulfate reflects processes operating at the nexus of sulfur, carbon, and oxygen cycles. However, knowledge of past seawater sulfate δ³⁴S values must be derived from proxy materials that are impacted differently by depositional and postdepositional processes. We produced new time series estimates for the δ³⁴S value of seawater sulfate by combining 6,710 published data from three sedimentary archives—marine barite, evaporites, and carbonate‐associated sulfate—with updated age constraints on the deposits. Robust features in multiple records capture temporal trends in the δ³⁴S value of seawater and its interplay with other Phanerozoic geochemical and stratigraphic trends. However, high‐frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints. The amount of noise, quantified from the variograms of each record, increases with age for all δ³⁴S proxies, indicating that postdepositional processes obscure detailed knowledge of seawater sulfate's δ³⁴S value deeper in time.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/104374, title ="Brachiopod δ³⁴S_(CAS) microanalyses indicate a dynamic, climate-influenced Permo-Carboniferous sulfur cycle", author = "Johnson, Daniel L. and Grossman, Ethan L.", journal = "Earth and Planetary Science Letters", volume = "546", pages = "Art. No. 116428", month = "September", year = "2020", doi = "10.1016/j.epsl.2020.116428", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200714-093504749", note = "© 2020 Elsevier B.V. \n\nReceived 6 December 2019, Revised 17 June 2020, Accepted 18 June 2020, Available online 7 July 2020. \n\nTed Present, Guillaume Paris, Jared Marske, Nathan Dalleska, Sharon Bone, and Courtney Roach provided valuable analytical support. We also thank the editor and two anonymous reviewers for thoughtful suggestions that substantially improved this manuscript. This work was supported by National Science Foundation (NSF) grants OCE-1559215, OCE-1737404, OCE-1450528, and MGG-1834492; NASA prime grant NNN12AA01C; Change Happens Foundation; and Grantham Foundation awards to JFA. DLJ additionally thanks the NSF for support through a Graduate Research Fellowship. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. \n\nThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.", revision_no = "10", abstract = "Early isotopic studies of sulfate in carbonate minerals (carbonate associated sulfate; CAS) suggested that carbonates can provide a reliable, well-dated archive of the marine sulfur cycle through time. However, subsequent research has shown that diagenetic alteration can impose highly heterogeneous CAS sulfur isotopic compositions (δ³⁴S_(CAS)) among different carbonate phases within sediments. Such alteration necessitates targeted sampling of well-preserved, primary carbonate phases. Here, we present a new record of Carboniferous and Early Permian brachiopod δ³⁴S_(CAS) generated from over 130 measurements of microsampled brachiopod shells. Our record refines existing brachiopod δ³⁴S_(CAS) records and confirms a large, ∼6.5‰ δ³⁴S_(CAS) decrease in the Early Carboniferous. Importantly, the record also features a novel 3–5‰ increase in δ³⁴S_(CAS) near the Serpukhovian-Bashkirian boundary (323.4 Ma) that coincides with carbonate δ¹³C and δ¹⁸O increases. Variability in δ³⁴S_(CAS) is minor both within (≤0.3‰) and among (≤2‰) individual co-depositional brachiopod specimens. A taxon-specific δ³⁴S_(CAS) offset is present one species (Composita subtilita) that also exhibits a δ¹³C offset, supporting the existence of biological “vital effects” on δ³⁴S_(CAS). Geologic evidence and mathematical modeling of the Permo-Carboniferous carbon and sulfur cycles suggest that changes in the burial ratio of organic carbon to pyrite sulfur (RC:S) are insufficient to explain the observed mid-Carboniferous δ³⁴S_(CAS) record. We find that changes in the ³⁴S depletion of pyrite relative to seawater sulfate (ε³⁴) or in the δ³⁴S of the input to the ocean (δ³⁴S_(in)) are also needed. Large additions of O₂ from organic carbon burial during the Permo-Carboniferous cannot be entirely compensated for with sulfur cycle changes; lower than modern late Visean pO₂ and/or additional O₂ sinks are needed to keep pO₂ at plausible levels. Based on the geologic context surrounding our record's mid-Carboniferous δ³⁴S_(CAS) increase, we advocate for simultaneous changes in pyrite burial, ε³⁴, and δ³⁴S_(in), driven by sea level or tectonically induced changes in environments of sulfur burial, as a viable mechanism to produce rapid seawater δ³⁴S changes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/104232, title ="Deposition of sulfate aerosols with positive Δ³³S in the Neoarchean", author = "Paris, Guillaume and Fischer, Woodward W.", journal = "Geochimica et Cosmochimica Acta", volume = "285", pages = "1-20", month = "September", year = "2020", doi = "10.1016/j.gca.2020.06.028", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200706-145806917", note = "© 2020 Published by Elsevier Ltd. \n\nReceived 21 February 2020, Revised 23 June 2020, Accepted 24 June 2020, Available online 3 July 2020. \n\nWe thank Dr. LaFlamme, Dr. Marin-Carbonne and one anonymous reviewer for their helpful reviews and Prof. Ono for handling the editorial process. Altogether, they helped us improve this article. We thank Prof. Beukes for his help in accessing the cores and for feedback on the geological context. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Financial support was received from the NSF (grant EAR-1349858 attributed to WWF and JFA). WWF acknowledges support from the Simons Foundation Collaboration on the Origins of Life. \n\nThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.", revision_no = "19", abstract = "Anomalous sulfur isotope compositions present in Archean rocks have been intensely scrutinized over the last 20\u202fyears because they record key aspects of Earth's atmospheric composition prior to the appearance of free molecular oxygen ca. 2.3 billion years ago. These isotopic compositions can be described as mass anomalous fractionations (MAF) and are produced in the atmosphere as UV light interacts with SO₂ molecules. Most interpretations suggest that atmospheric processes generate a reduced S-phase with a positive (³³S-enriched) MAF signature, as measured in pyrites, and an oxidized S-phase with a negative anomaly, as measured in bedded barite deposits. However, recent data for carbonate-associated sulfate (CAS) — a direct proxy for the isotopic composition of sulfur from seawater sulfate — in Neoarchean rocks showed no such negative values, but rather the opposite. To understand if the positive MAF anomalies we measured in Neoarchean CAS reflect secondary processes (diagenetic, metamorphic, handling) instead of original signals of Archean seawater sulfate, we collected additional sample suites with various degrees of preservation and metamorphic alteration across the Campbellrand-Malmani platform in South Africa. Results illustrate that within this comprehensive suite, less-altered samples all contain positive MAF values while secondary processes tend to either remove CAS from the sample and/or decrease the ³³S-enrichment. This positive MAF signal in sulfate is therefore reasonably interpreted as a primary depositional origin, and implies that the assumption that sulfate always carries a negative MAF anomaly throughout the Archean rock record needs to be reconsidered. Our CAS observations suggest that future experiments and calculations should also consider atmospheric and/or sulfur cycling processes that can produce oxidized sulfur with a positive MAF signature.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/103781, title ="An Atomic Force Microscopy Study of Calcite Dissolution in Seawater", author = "Dong, Sijia and Berelson, William M.", journal = "Geochimica et Cosmochimica Acta", volume = "283", pages = "40-53", month = "August", year = "2020", doi = "10.1016/j.gca.2020.05.031", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200609-071637828", note = "© 2020 Elsevier Ltd. \n\nReceived 8 November 2019, Revised 27 May 2020, Accepted 28 May 2020, Available online 6 June 2020. \n\nThis work was supported by NSF Ocean Acidification grants (numbers OCE1220600 and OCE1220302), USC Dornsife Doctoral Fellowship, and Grantham Foundation at Caltech. The authors would like to thank four anonymous journal reviewers, as well as the associate editor Dr. Oleg Pokrovsky, for their insightful comments and suggestions that helped to improve this manuscript. We thank Mina Hong, Zibo Li and Liang Zhao for helpful discussions on AFM operations. We also thank Josh West for his suggestions on the manuscript. \n\nThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.", revision_no = "20", abstract = "We present the first examination of calcite dissolution in seawater using Atomic Force Microscopy (AFM). We quantify step retreat velocity and etch pit density to compare dissolution in seawater to low ionic strength water, and also to compare calcite dissolution under AFM conditions to those conducted in bulk solution experiments (e.g. Subhas et al., 2015, Dong et al., 2018). Bulk dissolution rates and step retreat velocities are slower at high and mid-saturation state (Ω) values and become comparable to low ionic strength water rates at low Ω. The onset of defect-assisted etch pit formation in seawater is at Ω\u202f∼\u202f0.85 (defined as Ω_(critical)), higher than in low ionic strength water (Ω\u202f∼\u202f0.54). There is an abrupt increase in etch pit density (from ∼10⁶ cm⁻² to ∼10⁸ cm⁻²) occurring when Ω falls below 0.7 in seawater, compared to Ω\u202f∼\u202f0.1 in low ionic strength water, suggesting a transition from defect-assisted dissolution to homogeneous dissolution much closer to equilibrium in seawater. The step retreat velocity (v) does not scale linearly with undersaturation (1-Ω) across an Ω range of 0.4 to 0.9 in seawater, potentially indicating a high order correlation between kink rate and Ω for non-Kossel crystals such as calcite, or surface complexation processes during calcite dissolution in seawater.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/106974, title ="Marine20—The Marine Radiocarbon Age Calibration Curve (0–55,000 cal BP)", author = "Heaton, Timothy J. and Köhler, Peter", journal = "Radiocarbon", volume = "62", number = "4", pages = "779-820", month = "August", year = "2020", doi = "10.1017/rdc.2020.68", issn = "0033-8222", url = "https://resolver.caltech.edu/CaltechAUTHORS:20201209-105155334", note = "© 2020 by the Arizona Board of Regents on behalf of the University of Arizona. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. \n\nPublished online by Cambridge University Press: 12 August 2020. \n\nWe would like to thank Jeremy Oakley and Richard Bintanja for informative discussions during the development of this work. T.J. Heaton is supported by a Leverhulme Trust Fellowship RF-2019-140\\9, “Improving the Measurement of Time Using Radiocarbon”. M Butzin is supported by the German Federal Ministry of Education and Research (BMBF), as Research for Sustainability initiative (FONA); www.fona.de through the PalMod project (grant numbers: 01LP1505B, 01LP1919A). E. Bard is supported by EQUIPEX ASTER-CEREGE and ANR CARBOTRYDH. Meetings of the IntCal Marine Focus group have been supported by Collège de France. Data are available on the PANGAEA database at doi:10.1594/PANGAEA.914500. \n\nAuthor Contributions: TJH led this study and performed the Bayesian modeling. The general setup of Marine20 has been designed through discussions of the IntCal Marine Focus group led by EB. PK provided simulation results using the BICYCLE model, MB simulation results from the LSG OGCM. The manuscript has been written by TJH with large contributions from PK. RWR performed initial modeling tests incorporating pCO₂ data and recalculated ΔR values in the marine reservoir correction database with Marine20. All other co-authors provided expert knowledge and contributions to both discussions and the writing of the manuscript.", revision_no = "12", abstract = "The concentration of radiocarbon (¹⁴C) differs between ocean and atmosphere. Radiocarbon determinations from samples which obtained their ¹⁴C in the marine environment therefore need a marine-specific calibration curve and cannot be calibrated directly against the atmospheric-based IntCal20 curve. This paper presents Marine20, an update to the internationally agreed marine radiocarbon age calibration curve that provides a non-polar global-average marine record of radiocarbon from 0–55 cal kBP and serves as a baseline for regional oceanic variation. Marine20 is intended for calibration of marine radiocarbon samples from non-polar regions; it is not suitable for calibration in polar regions where variability in sea ice extent, ocean upwelling and air-sea gas exchange may have caused larger changes to concentrations of marine radiocarbon. The Marine20 curve is based upon 500 simulations with an ocean/atmosphere/biosphere box-model of the global carbon cycle that has been forced by posterior realizations of our Northern Hemispheric atmospheric IntCal20 ¹⁴C curve and reconstructed changes in CO₂ obtained from ice core data. These forcings enable us to incorporate carbon cycle dynamics and temporal changes in the atmospheric ¹⁴C level. The box-model simulations of the global-average marine radiocarbon reservoir age are similar to those of a more complex three-dimensional ocean general circulation model. However, simplicity and speed of the box model allow us to use a Monte Carlo approach to rigorously propagate the uncertainty in both the historic concentration of atmospheric ¹⁴C and other key parameters of the carbon cycle through to our final Marine20 calibration curve. This robust propagation of uncertainty is fundamental to providing reliable precision for the radiocarbon age calibration of marine based samples. We make a first step towards deconvolving the contributions of different processes to the total uncertainty; discuss the main differences of Marine20 from the previous age calibration curve Marine13; and identify the limitations of our approach together with key areas for further work. The updated values for ΔR, the regional marine radiocarbon reservoir age corrections required to calibrate against Marine20, can be found at the data base http://calib.org/marine/.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/102077, title ="Sulfur isotope fractionation between aqueous and carbonate-associated sulfate in abiotic calcite and aragonite", author = "Barkan, Yigal and Paris, Guillaume", journal = "Geochimica et Cosmochimica Acta", volume = "280", pages = "317-339", month = "July", year = "2020", doi = "10.1016/j.gca.2020.03.022", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200324-101330481", note = "© 2020 Published by Elsevier Ltd. \n\nReceived 25 July 2019, Accepted 16 March 2020, Available online 24 March 2020. \n\nWe thank Ziv Sade and Nir Galili for discussions. I.H. acknowledges support from a European Research Council Starting Grant (No. 337183). Part of this work was supported by the Tellus program from CNRS/INSU. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.", revision_no = "17", abstract = "Sulfate (SO₄²⁻) incorporated into calcium carbonate minerals enables measurements of sulfur (S) isotope ratios in carbonate rocks. This Carbonate Associated Sulfate (CAS) in marine carbonate minerals is thought to faithfully represent the S isotope composition of the seawater sulfate incorporated into the mineral, with little or no S isotope fractionation in the process. However, comparison between different calcifying species reveals both positive and negative S isotope fractionation between CAS and seawater sulfate, and a large range of S isotope ratios can be found within a single rock sample, depending on the component measured. To better understand the isotopic effects associated with sulfate incorporation into carbonate minerals, we precipitated inorganic calcite and aragonite over a range covering more than two orders of magnitude of sulfate concentration and precipitation rate. Coupled measurements of CAS concentration, S isotope composition and X-ray absorption near-edge spectra (XANES) permit characterization and explanation of the observed dependence of S isotope fractionation between CAS and aqueous sulfate (CAS-SO₄²⁻ isotope fractionation) on sulfate concentration and precipitation rate. In aragonite, the CAS-SO₄²⁻ isotope fractionation is 1.0±0.3‰ and independent of the sulfate (and CAS) concentration. In contrast, the CAS-SO₄²⁻ isotope fractionation in calcite covaries strongly with the sulfate concentration and weakly with the precipitation rate, between values of 1.3±0.1 and 3.1±0.6‰. We suggest that the correlation between aqueous sulfate concentration and CAS-SO₄²⁻– isotope fractionation in calcite reflects a dependence of the equilibrium S isotope fractionation on the concentration of CAS, through the effect of the sulfate impurity on the carbonate mineral’s energetic state.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/99432, title ="The carbonic anhydrase activity of sinking and suspended particles in the North Pacific Ocean", author = "Subhas, Adam V. and Adkins, Jess F.", journal = "Limnology and Oceanography", volume = "65", number = "3", pages = "637-651", month = "March", year = "2020", doi = "10.1002/lno.11332", issn = "0024-3590", url = "https://resolver.caltech.edu/CaltechAUTHORS:20191024-091357425", note = "© 2019 Association for the Sciences of Limnology and Oceanography. \n\nIssue Online: 05 March 2020; Version of Record online: 11 October 2019; Manuscript accepted: 22 August 2019; Manuscript revised:\n02 May 2019; Manuscript received: 12 December 2018. \n\nWe thank the entire science party and the R/V Kilo Moana crew on the CDisK‐IV cruise. In particular, we thank Doug Hammond and Alex Sessions for providing the in situ pumps, Nathan Kemnitz, Yi Hou, and Abby Lunstrum for help in working up pump filter and multicore samples, and Will Gray for helping to pick pteropod samples. We greatly thank Brian Hopkinson and two anonymous reviewers for their helpful and insightful comments and suggestions throughout the review process. This manuscript was significantly improved thanks to their efforts. We also acknowledge funding sources from NSF (OCE1220600 and OCE1220302), and the Resnick Sustainability Institute Graduate Fellowship for A.V.S. \n\nConflict of Interest: None declared.", revision_no = "19", abstract = "The enzyme carbonic anhydrase (CA) is crucial to many physiological processes involving CO₂, from photosynthesis and respiration, to calcification and CaCO₃ dissolution. We present new measurements of CA activity along a North Pacific transect, on samples from in situ pumps, sediment traps, discreet plankton samples from the ship's underway seawater line, plankton tows, and surface sediment samples from multicores. CA activity is highest in the surface ocean and decreases with depth, both in suspended and sinking particles. Subpolar gyre surface particles exhibit 10× higher CA activity per liter of seawater compared to subtropical gyre surface particles. Activity persists to 4700\u2009m in the subpolar gyre, but only to 1000\u2009m in the subtropics. All sinking CA activity normalized to particulate organic carbon (POC) follows a single relationship (CA/POC = 1.9\u2009±\u20090.2\u2009×\u200910⁻⁷ mol\u2009mol⁻¹). This relationship is consistent with CA/POC values in subpolar plankton tow material, suspended particles, and core top sediments. We hypothesize that most subpolar CA activity is associated with rapidly sinking diatom blooms, consistent with a large mat of diatomaceous material identified on the seafloor. Compared to the basin‐wide sinking CA/POC relationship, a lower subtropical CA/POC suggests that the inventory of subtropical biomass is different in composition from exported material. Pteropods also demonstrate substantial CA activity. Scaled to the volume within pteropod shells, first‐order CO₂ hydration rate constants are elevated ≥\u20091000× above background. This kinetic enhancement is large enough to catalyze carbonate dissolution within microenvironments, providing observational evidence for CA‐catalyzed, respiration‐driven CaCO₃ dissolution in the shallow North Pacific.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/101096, title ="Stable Isotope Analysis of Intact Oxyanions Using Electrospray Quadrupole-Orbitrap Mass Spectrometry", author = "Neubauer, Cajetan and Crémière, Antoine", journal = "Analytical Chemistry", volume = "92", number = "4", pages = "3077-3085", month = "February", year = "2020", doi = "10.1021/acs.analchem.9b04486", issn = "0003-2700", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200204-073456000", note = "© 2020 American Chemical Society. \n\nReceived: October 1, 2019; Accepted: January 21, 2020; Published: February 3, 2020. \n\nWe thank the reviewers for comments. We are grateful for assistance provided by Roxana Eggleston-Rangel and Brett Lomenick, and thank George Rossmann as well as the companies Peñoles, Crimidesa, Saltex, Airborne Industrial Minerals, and Searles Valley Minerals for providing sulfates. This study was using ESMS instrumentation at the Proteome Exploration Laboratory (supported by Beckman Institute, and NIH 1S10OD02001301). C.N. is a fellow of the Hanse-Wissenschaftskolleg. \n\nThe authors declare no competing financial interest.", revision_no = "20", abstract = "The stable isotopes of sulfate, nitrate, and phosphate are frequently used to study geobiological processes of the atmosphere, ocean, as well as land. Conventionally, the isotopes of these and other oxyanions are measured by isotope-ratio sector mass spectrometers after conversion into gases. Such methods are prone to various limitations on sensitivity, sample throughput, or precision. In addition, there is no general tool that can analyze several oxyanions or all the chemical elements they contain. Here, we describe a new approach that can potentially overcome some of these limitations based on electrospray hyphenated with Quadrupole Orbitrap mass spectrometry. This technique yields an average accuracy of 1–2‰ for sulfate δ³⁴S and δ¹⁸O and nitrate δ¹⁵N and δ¹⁸O, based on in-house and international standards. Less abundant variants such as δ¹⁷O, δ³³S, and δ³⁶S, and the ³⁴S–¹⁸O “clumped” sulfate can be quantified simultaneously. The observed precision of isotope ratios is limited by the number of ions counted. The counting of rare ions can be accelerated by removing abundant ions with the quadrupole mass filter. Electrospray mass spectrometry (ESMS) exhibits high-throughput and sufficient sensitivity. For example, less than 1 nmol sulfate is required to determine ¹⁸O/³⁴S ratios with 0.2‰ precision within minutes. A purification step is recommended for environmental samples as our proposed technique is susceptible to matrix effects. Building upon these initial provisions, new features of the isotopic anatomy of mineral ions can now be explored with ESMS instruments that are increasingly available to bioanalytical laboratories.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/94594, title ="Diagenetic controls on the isotopic composition of carbonate‐associated sulphate in the Permian Capitan Reef Complex, West Texas", author = "Present, Theodore M. and Gutierrez, Melissa", journal = "Sedimentology", volume = "66", number = "7", pages = "2605-2626", month = "December", year = "2019", doi = "10.1111/sed.12615", issn = "0037-0746", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190409-130651684", note = "© 2019 The Authors. Sedimentology © 2019 International Association of Sedimentologists. \n\nIssue Online: 19 November 2019; Version of Record online: 01 July 2019; Accepted manuscript online: 05 April 2019; Manuscript accepted: 03 April 2019; Manuscript received: 27 November 2018. \n\nFunding Information: the American Chemical Society Petroleum Research Fund New Directions. Grant Number: 53994‐ND2; Society for Sedimentary Geology (SEPM) Student Research Grant.", revision_no = "17", abstract = "Late Palaeozoic‐age strata from the Capitan Reef in west Texas show facies‐dependent heterogeneity in the sulphur isotopic composition of carbonate‐associated sulphate, which is trace sulphate incorporated into carbonate minerals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate‐associated sulphate. These processes variously modify the sulphur isotopic composition of incorporated sulphate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector inductively‐coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic textures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawater sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marine‐phreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic compositions of Permian seawater. Some early and all late meteoric calcite cements have carbonate‐associated sulphate with sulphur isotopic compositions distinct from that of Permian seawater. Detailed petrographic and sedimentary context for carbonate‐associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high‐energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater's sulphur isotopic composition.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/96927, title ="Calcite dissolution rates in seawater: Lab vs. in-situ measurements and inhibition by organic matter", author = "Naviaux, John D. and Subhas, Adam V.", journal = "Marine Chemistry", volume = "215", pages = "Art. No. 103684", month = "September", year = "2019", doi = "10.1016/j.marchem.2019.103684", issn = "0304-4203", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190708-154640644", note = "© 2019 Elsevier B.V. \n\nReceived 20 March 2019, Revised 15 June 2019, Accepted 4 July 2019, Available online 5 July 2019.", revision_no = "11", abstract = "Ocean acidification from fossil fuel burning is lowering the mean global ocean saturation state (Ω = [Ca^(2+)][CO_3^(2−)]/K_(sp)′), thus increasing the thermodynamic driving force for calcium carbonate minerals to dissolve. This dissolution process will eventually neutralize the input of anthropogenic CO_2, but the relationship between Ω and calcite dissolution rates in seawater is still debated. Recent advances have also revealed that spectrophotometric measurements of seawater pHs, and therefore in-situ Ωs, are systematically lower than pHs/Ωs calculated from measurements of alkalinity (Alk) and total dissolved inorganic carbon (DIC). The calcite saturation horizon, defined as the depth in the water column where Ω\u202f=\u202f1, therefore shifts by ~5–10% depending on the parameters used to calculate Ω. The “true” saturation horizon remains unknown. To resolve these issues, we developed a new in-situreactor and measured dissolution rates of ^(13)C-labeled inorganic calcite at four stations across a transect of the North Pacific Ocean. In-situ saturation was calculated using both Alk-DIC (Ω_((Alk, DIC))) and Alk-pH (Ω_((Alk, pH))) pairs. We compare in-situ dissolution rates with rates measured in filtered, poisoned, UV-treated seawater at 5 and 21\u202f°C under laboratory conditions. We observe in-situ dissolution above Ω_((Alk, DIC))\u202f=\u202f1, but not above Ω_((Alk, pH))\u202f=\u202f1. We emphasize that marine carbonate system equilibria should be reevaluated and that care should be taken when using proxies calibrated to historical Ω_((Alk, DIC)). Our results further demonstrate that calcite dissolution rates are slower in-situ than in the lab by a factor of ~4, but that they each possess similar reaction orders (n) when fit to the empirical Rate\u202f=\u202fk(1-Ω)^n equation. The reaction orders are n\u202f<\u202f1 for 0.8\u202f<\u202fΩ\u202f<\u202f1 and n\u202f=\u202f4.7 for 0\u202f<\u202fΩ\u202f<\u202f0.8, with the kink in rates at Ω_(crit)\u202f=\u202f0.8 being consistent with a mechanistic transition from step edge retreat to homogenous etch pit formation. We reconcile the offset between lab and in-situ rates by dissolving calcite in the presence of elevated orthophosphate (20\u202fμm) and dissolved organic carbon (DOC) concentrations, where DOC is in the form of oxalic acid (20\u202fμm), gallic acid (20\u202fμm), and D-glucose (100\u202fμm). We find that soluble reactive phosphate has no effect on calcite dissolution rates from pH\u202f5.5–7.5, but the addition of DOC in the form of D-glucose and oxalic acid slows laboratory dissolution rates to match in-situ observations, potentially by inhibiting the retreat rate of steps on the calcite surface. Our lab and in-situ rate data form an envelope around previous in-situ dissolution measurements and may be considered outer bounds for dissolution rates in low/high DOC waters. The lower bound (high DOC) is most realistic for particles formed in, and sinking out of, surface waters, and is described by R_((mol cm-2 s-1))\u202f=\u202f10^(–14.3±0.2)(1-Ω)^(0.11±0.1) for 0.8\u202f<\u202fΩ\u202f<\u202f1, and R_((mol cm-2 s-1))\u202f=\u202f10^(–10.8±0.4)(1-Ω)^(4.7±0.7) for 0\u202f<\u202fΩ\u202f<\u202f0.8. These rate equations are derived from in-situ measurements and may be readily implemented into marine geochemical models to describe water column calcite dissolution.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/96625, title ="Stratospheric eruptions from tropical and extra-tropical volcanoes constrained using high-resolution sulfur isotopes in ice cores", author = "Burke, Andrea and Moore, Kathryn A.", journal = "Earth and Planetary Science Letters", volume = "521", pages = "113-119", month = "September", year = "2019", doi = "10.1016/j.epsl.2019.06.006", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190621-110910481", note = "© 2019 Elsevier B.V. \n\nReceived 20 February 2019, Revised 4 June 2019, Accepted 5 June 2019, Available online 20 June 2019. \n\nThis research was funded by a Foster and Coco Stanback postdoctoral fellowship and a Marie Curie Career Integration Grant (CIG14-631752) to AB and a NSF-OCE grant 1340174 and NSF-EAR grant 1349858 to JFA. MS acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 820047). NSF-PLR grant 1204176 to JRM supported collection and analysis of the Tunu2013 core. We thank Sepp Kipfstuhl of the Alfred Wegener Institut for providing the B40 core. We thank Joel Savarino, an anonymous reviewer, and James Rae for comments on this manuscript.", revision_no = "11", abstract = "The record of volcanic forcing of climate over the past 2500 years is based primarily on sulfate concentrations in ice cores. Of particular interest are large volcanic eruptions with plumes that reached high altitudes in the stratosphere, as these afford sulfate aerosols the longest residence time in the atmosphere, and thus have the greatest impact on radiative forcing. Sulfur isotopes measured in ice cores can be used to identify these large eruptions because stratospheric sulfur is exposed to UV radiation, which imparts a time-evolving mass independent fractionation (MIF) that is preserved in the ice. However, sample size requirements of traditional measurement techniques mean that the MIF signal may be obscured, leading to an inconclusive result. Here we present a new method of measuring sulfur isotopes in ice cores by multi-collector inductively coupled plasma mass spectrometry, which reduces sample size requirements by three orders of magnitude. Our method allows us to measure samples containing as little as 10 nmol of sulfur, with a precision of 0.11‰ for δ^(34)S and 0.10‰ for Δ^(33)S, enabling a high-temporal resolution over ice core sulfate peaks. We tested this method on known tropical (Tambora 1815 and Samalas 1257) and extra-tropical (Katmai/Novarupta 1912) stratospheric eruptions from the Tunu2013 ice core in Greenland and the B40 ice core from Antarctica. These high-resolution sulfur isotope records suggest a distinct difference between the signatures of tropical versus extra-tropical eruptions. Furthermore, isotope mass balance on sulfate from extra-tropical eruptions provides a means to estimate the fraction of sulfate deposited that was derived from the stratosphere. This technique applied to unidentified eruptions in ice cores may thus improve the record of explosive volcanism and its forcing of climate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/97049, title ="A Southern Ocean Mechanism for the Interhemispheric Coupling and Phasing of the Bipolar Seesaw", author = "Thompson, Andrew F. and Hines, Sophia K.", journal = "Journal of Climate", volume = "32", number = "14", pages = "4347-4365", month = "July", year = "2019", doi = "10.1175/JCLI-D-18-0621.1", issn = "0894-8755", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190711-083803362", note = "© 2019 American Meteorological Society. \n\nManuscript received 20 September 2018, in final form 19 April 2019. \n\nWe thank Raffaele Ferrari and David Marshall for constructive comments to an earlier draft of this study and three anonymous reviewers for their insightful comments. We acknowledge helpful conversations with Paola Cessi, Malte Jansen, Brad Markle, Emily Newsom, Andrew Stewart, and Laure Zanna. We also thank Feng He for providing the TraCE simulation output. AFT received support from the David and Lucille Packard Foundation and from National Science Foundation (NSF) Grant OCE-1235488; SKH received support from NSF Grant P2C2-1503129 and the Lamont-Doherty Earth Observatory Postdoctoral Fellowship.", revision_no = "8", abstract = "The last glacial period is punctuated by abrupt changes in Northern Hemisphere temperatures that are known as Dansgaard–Oeschger (DO) events. A striking and largely unexplained feature of DO events is an interhemispheric asymmetry characterized by cooling in Antarctica during periods of warming in Greenland and vice versa—the bipolar seesaw. Methane-synchronized ice core records indicate that the Southern Hemisphere lags the Northern Hemisphere by approximately 200 years. Here, we propose a mechanism that produces observed features of both the bipolar seesaw and the phasing of DO events. The spatial pattern of sea ice formation and melt in the Southern Ocean imposes a rigid constraint on where water masses are modified: waters are made denser near the coast where ice forms and waters are made lighter farther north where ice melts. This pattern, coupled to the tilt of density surfaces across the Southern Ocean and the stratification of the ocean basins, produces two modes of overturning corresponding to different bipolar seesaw states. We present evolution equations for a simplified ocean model that describes the transient adjustment of the basin stratification, the Southern Ocean surface density distribution, and the overturning strength as the ocean moves between these states in response to perturbations in North Atlantic Deep Water formation, which we take as a proxy for Greenland temperatures. Transitions between different overturning states occur over a multicentennial time scale, which is qualitatively consistent with the observed Southern Hemisphere lag. The volume of deep density layers varies inversely with the overturning strength, leading to significant changes in residence times. Evidence of these dynamics in more realistic circulation models is discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/95957, title ="Dynamic intermediate waters across the late glacial revealed by paired radiocarbon and clumped isotope temperature records", author = "Hines, Sophia K. V. and Eiler, John M.", journal = "Paleoceanography and Paleoclimatology", volume = "34", number = "7", pages = "1074-1091", month = "July", year = "2019", doi = "10.1029/2019pa003568", issn = "2572-4517", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190530-102320622", note = "© 2019 American Geophysical Union. \n\nReceived 18 JAN 2019; Accepted 24 MAY 2019; Accepted article online 29MAY 2019; Published online 10 JUL 2019. \n\nWe would like to thank Andrew Thompson, Nivedita Thiagarajan, and Julia Gottschalk for helpful discussions. We also acknowledge constructive comments from two anonymous reviewers. S. K. V. H. received support from NSF Grants OCE‐1503129 and OCE‐1204211 and the Lamont‐Doherty Earth Observatory Postdoctoral Fellowship. All data are available in the supplemental tables and will be archived on the NOAA National Centers for Environmental Information.", revision_no = "35", abstract = "Paired radiocarbon and clumped isotope temperature records from U/Th‐dated Desmophyllum dianthus corals in the North Atlantic and Southern Ocean provide unique information about the history of intermediate waters (∼1,500–1,700 m) across the late glacial and deglaciation (∼35–10 ka). These measurements allow for the construction of radiocarbon‐temperature crossplots, which help to identify water mass endmembers at different times across the deglaciation. Radiocarbon and temperature values from the late glacial fall outside the range of modern ocean data from near the sample collection sites. In the North Atlantic, radiocarbon values tend to be much older than the modern, while in the Southern Ocean, they are more often younger than the modern. Reconstructed temperatures vary around respective modern ocean values; however, warm waters are observed at the Last Glacial Maximum and across the deglaciation in the north and south. We interpret our data in the context of the modern hydrography of the Western North Atlantic and Southern Ocean, and we draw upon direct comparisons between sediment core‐derived reconstructions of ocean circulation from the South Indo‐Pacific and our deep‐sea coral data from the Southern Ocean. Our North Atlantic data support accepted patterns of reduced North Atlantic Deep Water formation during Heinrich Stadials 1 and 2. In the Southern Ocean, deep‐sea coral populations respond to changes in ocean structure that are also reflected in a depth profile of δ^(13)C data from New Zealand, and data indicate that there was less influence of Pacific Deep Water between 1,500 and 1,700 m south of Tasmania across much of the deglaciation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/94070, title ="Aragonite dissolution kinetics and calcite/aragonite ratios in sinking and suspended particles in the North Pacific", author = "Dong, Sijia and Berelson, William M.", journal = "Earth and Planetary Science Letters", volume = "515", pages = "1-12", month = "June", year = "2019", doi = "10.1016/j.epsl.2019.03.016", issn = "0012-821X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190322-142418170", note = "© 2019 Elsevier B.V. \n\nReceived 8 November 2018, Revised 8 March 2019, Accepted 10 March 2019, Available online 22 March 2019. \n\nThis work was supported by NSF Ocean Acidification grants (numbers OCE1220600 and OCE1220302), USC Dornsife Doctoral Fellowship, Elizabeth and Jerol Sonosky Fellowship, the Sea Grant Fellowship, and the Resnick Sustainability Institute Graduate Fellowship. The authors would like to acknowledge editor Derek Vance, reviewer Jack Middelburg and another anonymous reviewer for their invaluable comments of the original manuscript. We thank the captain and crews on Kilo Moana for their assistance at sea. We also acknowledge Christopher Moore, Loraine Martell-Bonet for their help measuring pH and alkalinity during CDisK-IV; Yi Hou for leak-checking the Niskin Incubators by measuring dissolved Si concentrations; Doug Hammond for providing the in situ pumps; Johnny Stutsman and James Rae for their help deploying and recovering in situ pumps; as well as Abby Lunstrum and Huanting Hu for their help picking out swimmers from the sediment trap samples.", revision_no = "16", abstract = "The lack of consensus on CaCO_3 dissolution rates and calcite to aragonite production and export ratios in the ocean poses a significant barrier for the construction of global carbon budgets. We present here a comparison of aragonite dissolution rates measured in the lab vs. in situ along a transect between Hawaii and Alaska using a ^(13)C labeling technique. Our results show a general agreement of aragonite dissolution rates in the lab versus in the field, and demonstrate that aragonite, like calcite, shows a non-linear response of dissolution rate as a function of saturation state (Ω). Total carbon fluxes along the N. Pacific transect in August 2017, as determined using sediment traps, account for 11∼23 weight % of total mass fluxes in the upper 200 m, with a PIC (particulate inorganic carbon) /POC (particulate organic carbon) mole ratio of 0.2∼0.6. A comparison of fluxes at depths of 100 m and 200 m indicates that 30∼60% PIC dissolves between these depths with 20∼70% attenuation in POC fluxes. The molar ratio of PIC to POC loss is 0.29. The simultaneous loss of PIC and POC in the upper 200 m potentially indicates PIC dissolution driven by organic matter respiration, or metazoan/zooplankton consumption. The calcite/aragonite ratio in trap material is significantly lower in the subtropical gyre than in the subarctic gyre. Aragonite fluxes vary from 0.07 to 0.38 mmol m^(−2) day^(−1) at 100 m, and 0.06 to 0.24 mmol m^(−2) day^(−1) at 200 m along the North Pacific transect, with no specific trend over latitude. The identification of suspended PIC mineral phases by Raman spectroscopy shows the presence of aragonite below 3000 m in the subtropical gyre, but none in the subpolar gyre. These multiple lines of evidence suggest that predictions based on a strictly thermodynamic view of aragonite dissolution, combined with measured aragonite fluxes, underestimate observed alkalinity excess and measured PIC attenuation in sinking particles. Our measured aragonite flux combined with our inorganic dissolution rate only account for 9% and 0.2% of the excess alkalinity observed in the North Pacific (Feely et al., 2004), assuming aragonite sinking rates of 1 m day^(−1) and 100 m day^(−1), respectively. However, respiration-driven dissolution or metazoan/zooplankton consumption, indicated by the simultaneous attenuation of PIC and POC in sediment traps, is able to generate the magnitude of dissolution suggested by observed excess alkalinity.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/94932, title ="Spatial patterns of benthic silica flux in the North Pacific reflect upper ocean production", author = "Hou, Yi and Hammond, Douglas E.", journal = "Deep Sea Research Part 1", volume = "148", pages = "25-33", month = "June", year = "2019", doi = "10.1016/j.dsr.2019.04.013", issn = "0967-0637", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190424-111734892", note = "© 2019 Elsevier Ltd. \n\nReceived 9 August 2018, Revised 12 April 2019, Accepted 22 April 2019, Available online 24 April 2019.", revision_no = "10", abstract = "Diatoms are the dominant algal group that cycles dissolved silicic acid in the ocean; they also play an important role in the oceanic carbon cycle. It is therefore important to quantify the spatial distribution of silica cycling for defining global ocean biogeochemical cycles. On the research cruise CDisK-IV, water samples and sediment cores were collected at 5 stations along a North Pacific transect near 150ºW from 22ºN to 50ºN to evaluate benthic remineralization rates of biogenic silica (bSi). Two independent methods, core incubation and diffusive transport based on porewater profiles, were utilized to estimate benthic silicic acid fluxes, and these independent methods yield fluxes that agree within uncertainties. The benthic fluxes are reported as 0.04\u202f±\u202f0.01, 0.04\u202f±\u202f0.01, 0.05\u202f±\u202f0.01, 0.67\u202f±\u202f0.14, 0.40\u202f±\u202f0.08\u202fmmol Si m^(−2) day^(−1) for Stations 1 to 5, south to north, respectively. Burial fluxes were estimated using measurements of solid phase bSi in sediments and literature values of sediment accumulation rate. Burial efficiencies of bSi at all stations were <5% and show reasonable agreement with previous estimates. When burial rates were added to benthic fluxes to calculate rain rates, the rain observed under the subarctic gyre (Stations 4–5), was far larger than in the lower latitudes of the subtropics (Stations 1–3), corresponding to higher surface diatom productivity at higher latitudes. At the two northern stations, the bottom 500\u202fm of the water column shows a near-bottom increase in silicic acid that is consistent with the measured benthic flux and the estimated vertical eddy diffusivity. Above this horizon, water column density stratification increases and vertical diffusivity decreases, but the silicic acid gradient decreases. This reduction in gradient indicates that above this horizon, horizontal transport by deep waters, rather than vertical diffusion, becomes the dominant process removing the silicic acid released by benthic remineralization.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/92712, title ="A Cenozoic Record of Seawater Uranium in Fossil Corals", author = "Gothmann, Anne M. and Higgins, John A.", journal = "Geochimica et Cosmochimica Acta", volume = "250", pages = "173-190", month = "April", year = "2019", doi = "10.1016/j.gca.2019.01.039", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190206-103754732", note = "© 2019 Published by Elsevier Ltd. \n\nReceived 9 August 2018, Accepted 30 January 2019, Available online 6 February 2019. \n\nWe would like to thank Francois L.H. Tissot for helpful comments on multiple drafts of this manuscript as well as Associate Editor, Claudine Stirling, and an anonymous reviewer. We thank Stephen Cairns and Tim Coffer (Smithsonian Institution), Linda Ivany (Syracuse University), Roger Portell (Florida Museum of Natural History), Anne Cohen and Bill Thompson (WHOI), the USGS, and Gregory Dietl (Paleontological Research Institution) for loaning samples. Elizabeth Lundstrom (Princeton University) and Lindsey Hedges (California Institute of Technology) provided critical analytical support. We also thank Sarah Jane White (USGS), Francois Morel (Princeton University) and Will Amidon (Middlebury College) for helpful discussions that improved this manuscript.", revision_no = "19", abstract = "We measured U/Ca ratios, ^4He concentrations, ^(234)U/^(238)U, and ^(238)U/^(235)U in a subset of well-preserved aragonitic scleractinian fossil corals previously described by Gothmann et al. (2015). Comparisons of measured fossil coral He/U ages with the stratigraphic age demonstrate that well-preserved coral aragonite retains most or all of its radiogenic He for 10’s of millions of years. Such samples must be largely or entirely free of alteration, including neomorphism. Measurements of ^(234)U/^(238)U and ^(238)U/^(235)U further help to characterize the fidelity with which the original U concentration has been preserved. Analyses of fossil coral U/Ca show that the seawater U/Ca ratio rose by a factor of 4-5 between the Early Cenozoic and today. Possible explanations for the observed increase include (1) the stabilization of U in seawater due to an increase in seawater [CO_3^(2-)], and a resulting increase in UO_2-CO_3 complexation as originally suggested by Broecker (1971); (2) a decrease in the rate of low-temperature hydrothermal alteration from Early Cenozoic to present, leading to a diminished U sink and higher seawater [U]; or (3) a decrease in uranium removal in reducing sediments, again leading to higher seawater [U].", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/94246, title ="The role of the Southern Ocean in abrupt transitions and hysteresis in glacial ocean circulation", author = "Hines, Sophia K. V. and Thompson, Andrew F.", journal = "Paleoceanography and Paleoclimatology", volume = "34", number = "4", pages = "490-510", month = "April", year = "2019", doi = "10.1029/2018pa003415", issn = "2572-4517", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190328-112956704", note = "© 2019 American Geophysical Union. \n\nReceived 6 JUN 2018; Accepted 4MAR 2019; Accepted article online 15MAR 2019; Published online 5 APR 2019. \n\nWe would like to thank Raffaele Ferrari, Emily Newsom, Andrew Stewart, David Marshall, James Rae, and Andrea Burke for helpful discussions, and two anonymous reviewers, whose comments improved the manuscript. S. K. V. H. received support from NSF grants OCE‐1503129 and OCE‐1204211 and the Lamont‐Doherty Earth Observatory Postdoctoral Fellowship. A. F. T. received support from the David and Lucille Packard Foundation and from NSF grant OCE‐1235488. J. F. A. received support from NSF grants OCE‐1503129, OCE‐1737404, and OCE‐1450528. The model code used for this paper will be available on GitHub at the https://github.com/shiness11/DynBoxTwoB4L website.", revision_no = "24", abstract = "High‐latitude Northern Hemisphere climate during the last glacial period was characterized by a series of abrupt climate changes, known as Dansgaard‐Oeschger (DO) events, which were recorded in Greenland ice cores as shifts in the oxygen isotopic composition of the ice. These shifts in inferred Northern Hemisphere high‐latitude temperature have been linked to changes in Atlantic meridional overturning strength. The response of ocean overturning circulation to forcing is non‐linear and a hierarchy of models have suggested that it may exist in multiple steady state configurations. Here, we use a time‐dependent coarse‐resolution isopycnal model with four density classes and two basins, linked by a Southern Ocean to explore overturning states and their stability to changes in external parameters. The model exhibits hysteresis in both the steady‐state stratification and overturning strength as a function of the magnitude of North Atlantic Deep Water (NADW) formation. Hysteresis occurs as a result of two non‐linearities in the model‐‐‐the surface buoyancy distribution in the Southern Ocean and the vertical diffusivity profile in the Atlantic and Indo‐Pacific basins. We construct a metric to assess circulation configuration in the model, motivated by observations from the Last Glacial Maximum, which show a different circulation structure from the modern. We find that circulation configuration is primarily determined by NADW density. The model results are used to suggest how ocean conditions may have influenced the pattern of DO events across the last glacial cycle.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/92252, title ="Precise determination of equilibrium sulfur isotope effects during volatilization and deprotonation of dissolved H_2S", author = "Sim, Min Sub and Sessions, Alex L.", journal = "Geochimica et Cosmochimica Acta", volume = "248", pages = "242-251", month = "March", year = "2019", doi = "10.1016/j.gca.2019.01.016", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190114-132114245", note = "© 2019 Published by Elsevier Ltd. \n\nReceived 16 June 2018, Revised 29 December 2018, Accepted 9 January 2019, Available online 14 January 2019. \n\nThis research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2018R1D1A1B07050970) and an Agouron Geobiology Fellowship to MSS, Gordon and Betty Moore Foundation Grant GBMF 3306 to VJO and ALS, NSF award OCE-1436566 to ALS and NSF award OCE-1340174 to JFA. The authors are grateful to Guillaume Paris for assistance for isotope analysis. We also thank Daniel Eldridge, Boswell Wing, and an anonymous reviewer for constructive comments on an earlier version of this manuscript.", revision_no = "19", abstract = "Sulfide (H_2S, HS^−, and S^(2−)) is ubiquitous in marine porewaters as a result of microbial sulfate reduction, constituting the reductive end of the biogeochemical sulfur cycle. Stable isotopes have been widely used to constrain the sulfur cycle, because the redox transformations of sulfur compounds, such as microbial sulfate reduction, often exhibit sizable kinetic isotope effects. In contrast to sulfate ion (SO_4^(2−)), the most abundant form of dissolved sulfur in seawater, H2S is volatile and also deprotonated at near neutral pH. Equilibrium isotope partitioning between sulfide species can therefore overlap with kinetic isotope effects during reactions involving sulfide as either reactant or intermediate. Previous experimental attempts to measure equilibrium fractionation between H_2S and HS− have reached differing results, likely due to solutions of widely varying ionic strength. In this study, we measured the sulfur isotope fractionation between total dissolved sulfide and gaseous H2S at 20.6\u202f±\u202f0.5\u202f°C over the pH range from 2 to 8, and calculated the equilibrium isotope effects associated with deprotonation of dissolved H_2S. By using dilute solutions of Na2S, made possible by the improved sensitivity of mass spectrometric techniques, uncertainty in the first dissociation constant of H2S due to ionic strength could be better controlled. This in turn allowed us to close sulfur isotope mass balance for our experiments and increase the accuracy of the estimated fractionation factor. At equilibrium, aqueous H2S was enriched in ^(34)S by 0.7‰ and 3.1‰ relative to gaseous H_2S and aqueous HS−, respectively. The estimated fractionation between aqueous H_2S and HS^− lies between two earlier experimental reports, but agrees within the uncertainty of the measurements with a recent theoretical calculation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/91499, title ="Temperature Dependence of Calcite Dissolution Kinetics in Seawater", author = "Naviaux, John D. and Subhas, Adam V.", journal = "Geochimica et Cosmochimica Acta", volume = "246", pages = "363-384", month = "February", year = "2019", doi = "10.1016/j.gca.2018.11.037", issn = "0016-7037", url = "https://resolver.caltech.edu/CaltechAUTHORS:20181205-100011581", note = "© 2018 Published by Elsevier Ltd. \n\nReceived 1 August 2018, Revised 26 November 2018, Accepted 27 November 2018, Available online 5 December 2018. \n\nWe would like to thank the anonymous journal reviewers as well as Oleg Pokrovsky and Henry Teng for the insightful comments and suggestions they gave that helped to improve this manuscript. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. 1745301, as well as NSF grant Numbers OCE1220302, OCE1559004 and 1559215. John Naviaux and Adam Subhas would also like to thank the Resnick Sustainability Institute at Caltech for fellowship support.", revision_no = "21", abstract = "Knowledge of calcite dissolution kinetics in seawater is a critical component of our understanding of the changing global carbon budget. Towards this goal, we provide the first measurements of the temperature dependence of calcite dissolution kinetics in seawater. We measured the dissolution rates of ^(13)C-labeled calcite in seawater at 5, 12, 21, and 37°C across the full range of saturation states (0 < Ω = Ca^(2+)[CO_3^(2-)/Ksp'< 1). We show that the dissolution rate is non-linearly dependent on Ω and that the degree of non-linearity both increases with temperature, and changes abruptly at “critical” saturation states (Ω_(crit_). The traditional exponential rate law most often utilized in the oceanographic community, R=k(1-Ω)^n, requires different fits to k and n depending upon the degree of undersaturation. Though we calculate a similar activation energy to other studies far from equilibrium (25±2 kJ/mol), the exponential rate law could not be used to mechanistically explain our near equilibrium results. We turn to an alternative framework, derived from crystal nucleation theory, and find that our results are consistent with calcite dissolution kinetics in seawater being set by the retreat of pre-existing edges/steps from Ω=1-0.9, defect-assisted etch pit formation from Ω=0.9-0.75, and finally homogenous etch pit formation from Ω=0.75-0. The Ω_(crit) s for each mechanism are shifted significantly closer to equilibrium than they occur in dilute solutions, such that ocean acidification may cause marine carbonates to enter faster dissolution regimes more readily than would be expected from previous studies. We use the observed temperature dependence for each dissolution mechanism to calculate step kinetic coefficients (β, cm/s), densities of active nucleation sites (n_s, sites/m^2), and step edge free energies (α, mJ/m^2). Homogenous dissolution is well explained within the surface nucleation framework, but defect-assisted dissolution is not. Dissolution is initiated via step-propagation at all temperatures, but the defect-assisted mechanism is skipped over at 5°C, potentially due to a lack of nucleation sites. The surface nucleation framework enhances our understanding of calcite dissolution in seawater, but our results suggest that a complete theory will also need to incorporate the role of solution/surface speciation and complexation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/92242, title ="Role of APS reductase in biogeochemical sulfur isotope fractionation", author = "Sim, Min Sub and Ogata, Hideaki", journal = "Nature Communications", volume = "10", pages = "Art. No. 44", month = "January", year = "2019", doi = "10.1038/s41467-018-07878-4", issn = "2041-1723", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190114-083925177", note = "© 2018 The Author(s).\nThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.\n\nReceived 29 June 2018; Accepted 29 November 2018; Published\n09 January 2019.\n\nData availability:\nData supporting the findings of this study are available within the paper and in the supplementary information file or are available from the corresponding author upon reasonable request.\n\n\nThis work was supported by the Research Resettlement Fund for the new faculty of Seoul National University to M.S.S., the NASA Research Opportunities in Space and Earth Sciences grant award number NNX14AO48G to S.E.M. and V.J.O., JSPS KAKENHI Grant Number 10751084 to S.E.M., and the Gordon and Betty Moore Foundation Grant GBMF 3306 to V.J.O. and A.L.S. This research was a part of the project titled ‘Understanding the deepsea biosphere on seafloor hydrothermal vents in the Indian Ridge (20170411)’, funded by the Ministry of Oceans and Fisheries, Korea. We are grateful for insightful and helpful conversations with Boswell A. Wing, David T. Johnston, David A. Fike, and Itay Halevy. We are especially grateful to Tatsuhiko Yagi and Yoshiki Higuchi for helping with initiating collaboration.\n\nContributions:\nM.S.S. and S.E.M. devised the study. H.O. and W.L. purified APS reductase, and M.S.S. executed enzymatic assay and sulfur isotope measurements. M.S.S. and S.E.M. wrote the first draft of the manuscript, and J.F.A., A.L.S., and V.J.O. contributed to interpretation and writing.\n\nCompeting interests:\nThe authors declare no competing interests.\n", revision_no = "20", abstract = "Sulfur isotope fractionation resulting from microbial sulfate reduction (MSR) provides some of the earliest evidence of life, and secular variations in fractionation values reflect changes in biogeochemical cycles. Here we determine the sulfur isotope effect of the enzyme adenosine phosphosulfate reductase (Apr), which is present in all known organisms conducting MSR and catalyzes the first reductive step in the pathway and reinterpret the sedimentary sulfur isotope record over geological time. Small fractionations may be attributed to low sulfate concentrations and/or high respiration rates, whereas fractionations greater than that of Apr require a low chemical potential at that metabolic step. Since Archean sediments lack fractionation exceeding the Apr value of 20‰, they are indicative of sulfate reducers having had access to ample electron donors to drive their metabolisms. Large fractionations in post-Archean sediments are congruent with a decline of favorable electron donors as aerobic and other high potential metabolic competitors evolved.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/88997, title ="CO_2 storage and release in the deep Southern Ocean on millennial to centennial timescales", author = "Rae, J. W. B. and Burke, A.", journal = "Nature", volume = "562", number = "7728", pages = "569-573", month = "October", year = "2018", doi = "10.1038/s41586-018-0614-0", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180821-155408115", note = "© 2018 Springer Nature Limited. \n\nReceived 06 March 2018; Accepted 29 August 2018; Published 24 October 2018. \n\nData availability: The data produced in this study are available in Extended Data Tables and will also be made available at the NOAA (https://www.ncdc.noaa.gov/paleo/study/25230) and Pangaea data repositories. \n\nThis work was supported by NERC Standard Grant NE/N003861/1 to J.W.B.R. and L.F.R., an NOAA Climate and Global Change VSP Fellowship to J.W.B.R, NERC Standard Grant NE/M004619/1 to A.B. and J.W.B.R., a NERC Strategic Environmental Science Capital Grant to A.B. and J.W.B.R., Marie Curie Career Integration Grant CIG14-631752 to A.B., an ERC consolidator grant to L.F.R., NSF grant OCE-1503129 to J.F.A., and NERC studentships to B.T. and E.L. \n\nReviewer information: Nature thanks C. Buizert and the other anonymous reviewer(s) for their contribution to the peer review of this work.", revision_no = "61", abstract = "The cause of changes in atmospheric carbon dioxide (CO_2) during the recent ice ages is yet to be fully explained. Most mechanisms for glacial–interglacial CO_2 change have centred on carbon exchange with the deep ocean, owing to its large size and relatively rapid exchange with the atmosphere. The Southern Ocean is thought to have a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region. However, it is difficult to reconstruct changes in deep Southern Ocean carbon storage, so few direct tests of this hypothesis have been carried out. Here we present deep-sea coral boron isotope data that track the pH—and thus the CO_2 chemistry—of the deep Southern Ocean over the past forty thousand years. At sites closest to the Antarctic continental margin, and most influenced by the deep southern waters that form the ocean’s lower overturning cell, we find a close relationship between ocean pH and atmospheric CO_2: during intervals of low CO_2, ocean pH is low, reflecting enhanced ocean carbon storage; and during intervals of rising CO_2, ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial- to centennial-scale) decreases in pH during abrupt increases in CO_2, reflecting the rapid transfer of carbon from the deep ocean to the upper ocean and atmosphere. Our findings confirm the importance of the deep Southern Ocean in ice-age CO_2 change, and show that deep-ocean CO_2 release can occur as a dynamic feedback to rapid climate change on centennial timescales.", }